The present invention relates to a burner for operating a heat generator, the burner having a swirl generator and a mixing section downstream of the swirl generator.
In order to achieve high efficiencies and high power densities, the principle of staged combustion is employed in modem gas turbines. With staged combustion, a hot gas is generated in a high-pressure combustion chamber, preferably equipped with premixing burners (see EP-0 321 809 B1). After partial expansion in a first high-pressure turbine, this hot gas flows into a second combustion chamber (reheat combustion chamber), in which the partially expanded hot gas is reheated to the maximum turbine entry temperature, so that such a gas turbine is based on sequential combustion. Such a gas turbine is disclosed in U.S. Pat. Nos. 5,454,220, and 5,577,378, and EP-0 620 362 A1, all of which are incorporated herein by reference.
The reheat combustion chamber has a configuration which is shown and described in more detail in the publications EP-0 745 809 A1 and EP-0 835 996 A1, these publications likewise being incorporated herein by reference.
This combustion chamber is of annular configuration and has, essentially, a series of individual adjacent transfer ducts which form the individual burners of this combustion chamber. These individual burners have an almost rectangular cross section. Mixing occurs in these individual burners between the fuel and the oxygen-containing partially expanded hot gas, called combustion air, which comes from the first turbine. This mixing takes place by means of a plurality of longitudinal vortex generators, also called vortex generators, which are attached to the cold walls of the mixing section and are themselves cooled. These longitudinal vortex generators (see EP-0 745 809 A1) generate, on their surfaces, a flow separation and deflection which finally leads to the formation of longitudinal vortices. The rectangular mixing duct is almost filled by a plurality of these longitudinal vortices. It has been found that such a configuration can lead to shortcomings for quite specific types of operation and operating parameters:
a) A substantial recooled air mass flow is required for cooling the mixing section and the longitudinal vortex generator.
b) The longitudinal vortices generated cannot completely cover the flow cross section, particularly in the right angled corners of the mixing duct.
c) During the generation of the longitudinal vortices, kinetic energy is dissipated into turbulence in the separation regions of the longitudinal vortex generators and only a part of the kinetic energy is converted into the longitudinal vortex motion.
d) After the step-shaped expansion into the combustion chamber, the flow attaches to the combustion chamber wall only after a substantial distance, i.e. undesirable dead water and reverse flow regions occur after the step discontinuity.
In view of the above-discussed problems with convention al premixing burners, and according to an embodiment of the present invention, mixing of the combustion air, i.e. the partially expanded hot gases and fuel, is effected in a round mixing section, which mixing section is filled by a single swirled flow. A conical swirl generator is provided, with the conical swirl generator including a plurality of partial bodies, or swirl generator vanes, which swirl the combustion air. The swirl generator vanes are installed for swirl generation in the entry cross section, whose rectangular shape is retained. The entry slots formed by the swirl generator vanes are present between the individual partial bodies of the swirl generator and are preferably of constant slot width, but they can have a variable slot width along their length. Four inlet slots are preferably provided, although embodiments with a different number of slots are also possible. A swirl generator according to an embodiment of the present invention has some similarities and a number of significant differences relative to a swirl generator such a s is described in DE-44 35 266 A1. As an example, the swirl generator according to the invention is installed in an initially rectangular duct, with this duct merging over the length of the swirl generator into a duct with approximately square or round cross section. This design helps to ensure a clean incident flow onto the swirl generator inlet slots, thereby avoiding any possible flow separations.
A fuel distribution system for gaseous and/or liquid fuels is integrated into the swirl generator, the fuel distribution taking place along the inlet slots, as is disclosed in the last-mentioned publication, through the trailing edges of the swirl generator vanes or by axial injection of the fuel from the vane surface. The injection of a low calorific value or medium calorific value fuel (Lbtu, Mbtu) is generally involved. The fuel can, if necessary, be surrounded by a cold carrier-air flow or inert gas flow in order to avoid premature ignition of the fuel. Liquid fuels are advantageously injected at the upstream end of the swirl generator through a plurality of injection jets. In accordance with the invention, it has been found that it is extremely advantageous for the number of the injection jets to agree with the number of inlet slots in the swirl generator or the number of swirl generator vanes.
A free flow duct, which prevents a reverse flow zone from becoming established in the mixing tube, is located on the axis of the swirl generator. The supply of liquid fuel must therefore take place external to the axis, preferably by means of the multiple nozzle system already mentioned above.
In order to protect the swirl generator vanes from oxidation attacks, as a consequence of the hot inlet temperatures, these swirl generator vanes can be manufactured from a ceramic material and/or can be provided with an internal air cooling system. The design and manufacture of the air-cooled swirl generator vanes follows from the rules known from cooled turbine blades; the heat transfer coefficients, however, are substantially smaller because of the lower flow velocities as compared with rotor blades or guide vanes in the turbine.
Downstream of the swirl generator, the flow is guided within a cylindrical mixing tube. The transition from the swirl generator to the mixing tube is preferably designed in such a way that the flow cross-sectional area is almost constant so that no flow separations occur. This can be achieved either by means of a specially shaped transition piece or by immersing the swirl generator vanes in the cylindrical duct. A further possibility can include providing the swirl generator vanes with a trailing edge which is cut away in the axial direction. In this arrangement, the length of the mixing tube is selected in such a way that the fuel injection cannot, before the end of the mixing tube, exceed the self-ignition time of the selected fuels. The length of the mixing section can vary between zero and two burner diameters depending on the size of the burner and as a function of the burner pressure loss selected.
Flame flashback blockage films can be introduced in order to avoid flashback of the flame into the boundary layers which build up on the transition piece and the mixing tube.
A separating edge, which can be configured in various ways, can be applied to the downstream end of the mixing tube. This separating edge stabilizes the boundary layer by means of the Coanda effect, due to a convex curvature of the end part, and deflects the whole of the flow toward the outside. This has, on the one hand, the effect that the flow in the combustion chamber attaches to the wall more rapidly, and is decelerated more rapidly, so that a turbulent flame front can be established. On the other hand, a part of the dynamic pressure can be recovered in the manner of a diffuser effect because of the deceleration at the end of the mixing tube.
The swirl strength can be made sufficiently strong for a reverse-flow region to occur on the axis downstream of the mixing section. The swirl strength should, however, be of such a strength that the flow downstream of the mixing tube and within one mixing tube diameter on the axis is only decelerated to velocities smaller than the combustion chamber velocity averaged over the cross section. The turbulent velocity fluctuations generated during this loss-affected deceleration serve to stabilize the flame.
The supply of fuel to the swirl generator takes place by means of a fuel and cooling air supply system extending radially relative to the burner axis. The swirl generators can be fastened to the fuel supply system and, together with the latter, can be removed radially from the gas turbine without the need to lift the casing.
Some important advantages of the invention may be seen in the fact that:
a) the surface area sensitive to flame flash-back is minimized by the design, according to the invention, of the mixing section as a cylindrical tube; by this means, the cooling and filling air required for flash-back blockage and for cooling the wall is reduced and, therefore, the overall process is optimized;
b) it is possible to fill the complete mixing section in an optimum manner with a single longitudinal vortex by means of the design, according to the invention, of the mixing section as a cylindrical tube;
c) the injection of the fuel along the inlet slot permits satisfactory fine distribution, by which means the mixing section necessary after the swirl generator is minimized;
d) the swirl generation takes place in a low-loss manner, i.e. no separation regions and no total-pressure loss zones are generated in the case of a design according to the invention. As a result, the pressure loss coefficient of the burner is small in relation to the effective flow cross section and only a little flame-stabilizing turbulence is present in the mixing section so that flame flash-back is avoided even at low flow velocities;
e) because of the strong flow deceleration downstream of the mixing tube, it is possible to increase the opening ratio, namely the ratio of the mixing tube cross section to the proportional combustion chamber cross section to values ranging from more than 4 to at least 10. Because of this, it is possible to construct an essentially shorter combustion chamber while retaining the same residence time and, therefore, satisfactory burn-out;
f) the swirl generator and the fuel injection can be structurally designed in such a way that they can be removed radially outward without removing the gas turbine casing.
Replacement swirl generators can therefore be easily fitted and a change to different fuels or fuel injection systems can be effected more easily.